In order to perform critical scientific measurements which depend for accuracy upon the quality and character of light beams, it is essential to accurately determine the vergence of the beam. Instruments having special detectors have been used. Heretofore, this has been done in the visible spectrum with instruments such as Continental Optical Company's collimation tester which includes a wedged shear plate and a screen on which light patterns can be optically displayed and measured, but such instruments are not adapted to the infrared range. Moreover, although some prior instruments, such as energy detectors, have used detectors on which bismuth spots have been applied, none of these instruments has applied bismuth in the manner described herein.